Study on the reaction mechanism and kinetics of limestone hydrogenation by micro fluidized bed: Effect of H2 concentration and natural limestone

Abstract

Limestone decomposition is the first step in cement production, which produces a significant amount of CO₂ and poses a significant challenge to achieve carbon neutrality. Hydrogenation of limestone can produce CO or CH₄ instead of CO₂, which can be considered as a new way of carbon capture, making it a promising method for carbon emission reduction. In this work, pure CaCO₃ and several natural limestone samples were hydrolyzed under varying H₂ concentration using a micro fluidized bed (MFB) reactor combined with online mass spectrometry to reveal the mechanism and kinetics of limestone hydrogenation.

The main gases produced by hydrogenation at 1 atm are CO and CO₂. The CO₂ comes from the calcination of CaCO₃. The CO comes from 2 steps: the first step is the in-situ hydrogenation of CaCO₃ and the second step is the Reverse Water Gas Shift (RWGS) reaction. The activation energy (Ea) of CO₂ formation in H₂ atmosphere is lower than in Ar atmosphere. However, there is no obvious effect of different H₂ concentrations on the Ea of CO₂ formation. The Ea of CO in situ formation is 72.70 KJ/mol, 54.53 KJ/mol, 71.34 KJ/mol and 60.29 KJ/mol in 10%, 30%, 50% and 70% H₂ atmosphere, respectively. The H₂ concentration also has no significant effect on the in-situ CO evolution. However, the H₂ concentration can affect the Ea of CO produced by RWGS. The Ea is 75.67 KJ/mol, 167.59 KJ/mol and 221.47 KJ/mol in 10%, 30% and 50% H₂ atmosphere, and this reaction doesn't occur in 70% H₂ atmosphere. Compared with the pure CaCO₃, the hydrogenation of limestone can produce more CO and less CO₂. In limestone, impurity elements are the main factor affecting the reaction kinetics. Transition metals can increase the rate of CO₂ production, but have no apparent effect on CO. The CO₂ yield of high impurity limestone is higher than that of limestone with low impurities. Transition metals can also reduce the Ea of CO₂ formation and the RWGS reaction. In the 50% H₂ atmosphere, the Ea of CO₂ formation is 88.87 KJ/mol and the Ea of CO from RWGS is 137.60 KJ/mol. However, under the same conditions, the Ea of pure CaCO₃ is 126.91 KJ/mol and 221.47 KJ/mol.